Spanish Language Stack Exchange is a question and answer site for students, teachers, and linguists wanting to discuss the finer points of the Spanish language. It's 100% free, no registration required.

Please add at least an example of a sentence showing your doubt ("pero" has a few possible meanings, one of them was "sino", but it is now obsolete lema.rae.es/drae/?val=pero)
–
belisariusJul 1 '13 at 21:29

"Sino" can often be translated as instead or rather (where these words function as "but").

"Sino" is a more connected conjuntion: the words that follow complete the sentence, it's syntactically related and it's contruction depends on the opening. By contrast, "Pero" opens a new independent clause. See the above examples. Quick rule: only before a "pero" we can replace the comma by a semicolon or a period ("Él tiene una camisa. Pero no es roja.").

So would "I haven't come to be served, but I have come to serve" use pero or sino? It doesn't fit your rule that the second clause is independent, but it fits the other rules.
–
Flimzy♦Jul 2 '13 at 23:12

I don't see it that way. The correct/natural form is "I haven't come to be served, but to serve". In any case, the second part, what follows the "but", mimics syntantically the first (just toggling the negative to a positive), it's not an independent construct.
–
leonbloyJul 2 '13 at 23:59

I wonder if you didn't understand my question. Sometimes, for stylistic reasons or emphasis, for example, someone would repeat the entire phrase. I'm asking if sino would still be appropriate then? Another English example might be, "I don't like cats, but I do like dogs." Is sino still appropriate, or would pero be used, since there is an independent clause?
–
Flimzy♦Jul 3 '13 at 0:11

Sino is a conjunction, and is used exactly the same as pero, except that it is used in cases of negation. It can be thought to roughly translate to the English terms however or rather, but is more specific than the English terms.

To use sino, there must be a negative word (such as no) preceding the conjunction

Examples:

I don't want to go to the movies, rather/however/but I want to go home.